My gf late her period about 3 weeks. i have to mention that she didnt have sex before me half year. so is it necessary pregnant or just hormone?

Either is possible: Pregnancy is possible but is not the only cause of a late period. If in doubt, check a pregnancy test and, if it is negative and her period still hasn't started, repeat it in 3-5 days. If it is still negative at that point she needs to be seen to find out why and that is a good time to also start a reliable method of contraception unless you want a baby. Best wishes.